The present invention relates to an system and method for reflowing solder to electrically connect electronic components to a flexible substrate having a low softening temperature.
It is well known in the art to mount electronic components to rigid and flexible printed circuit boards. Typically, solder paste is applied to conductor pad regions on the rigid or flexible substrate. Components are then placed with their terminals contacting the solder paste in the pad regions. The substrate is then exposed to relatively high temperatures to activate the solder paste which melts and then solidifies to bond and electrically connect the components onto the substrate. The flexible substrates are typically made from polyimide, which exhibits good stability when exposed to high temperatures. Many film materials, including polyesters, have not been used satisfactorily for surface mount components primarily because they exhibit inadequate heat resistance and dimensional stability when exposed to the temperatures required for solder reflow.
A technique for mounting components onto flexible polyester substrates with low softening temperatures is taught by Annable in U.S. Pat. No. 5,898,992. The flexible substrate is fixed to a carrier support member. A cover is placed over the substrate. The cover has openings corresponding to component locations and with the carrier forms a carrier assembly. Solder paste is applied to the conductor regions of the substrate having component pads. Electronic components are then placed on the substrate with their terminals in contact with the solder paste. The carrier assembly is then pre-heated in a reflow oven to a temperature below the melting point of the solder paste. The assembly is then subjected to a rapid rise in temperature utilizing a supplemental heat source such as a heated gas jet. The cover shields the substrate from the high reflow temperatures and minimizes distortion of the flexible substrate during reflow.
While the prior art teaching achieve their intended purpose significant improvements are needed. For example, it would be desirable to eliminate the need for a special cover for shielding specific areas of the substrate from the heat generated by the gas jet.
In an embodiment of the present invention a system for reflowing solder to interconnect a plurality of electronic components to a substrate is provided. The system includes an oven for preheating the substrate and the plurality of electronic components disposed thereon, a supplemental heat source positioned within the oven for providing additional heat energy to reflow the solder, wherein the supplemental heat source creates a stream of hot gas that flows transversely across the substrate, and a pallet for supporting and absorbing heat from the substrate.
In another embodiment of the present invention the supplemental heat source is a nozzle positioned within the oven, wherein the nozzle has a plurality of vanes for directing hot gas transversely across the substrate.
In still another embodiment the pallet and cover are made of a suitable conductive material with good thermal diffusivity, such as a heat resistant carbon fiber composite. Other materials for the pallet include a thin layer of copper backed with a glass-filled epoxy such as FR4.
Preferably, the circuit conductors on the substrate are copper. Selected regions of the conductors referred to as component pads are provided with a surface finish such as tin or immersion silver to enhance the ease of soldering to the pads. The spaces between the conductor regions of the substrate may be filled with electrically isolated regions of copper having the same thickness as the conductor regions. These copper areas further shield the substrate during reflow by selectively absorbing heat during the reflow process.
Components may be mounted on both the top and bottom sides of the substrate. For such a substrate, the reflow process is repeated for the second side. The pallet has appropriate cavities to accommodate the components on the first side of the substrate.
The flexible circuit may comprise more than two layers of circuit conductors, commonly referred to as multi-layer circuits. For these circuits, two or more layers of the substrate film are used and bonded together with a suitable adhesive to form four or more conductor layers.
Any convenient solder paste formulation may be used provided that it can be activated at a suitable temperature. In an embodiment of the present invention a suitable solder paste has a melting temperature of 183 degrees centigrade with a composition of 63 percent tin and 37 percent lead. Other solder paste compositions include lead-free solders that are alloys of tin, silver and copper, but exhibit higher melting temperatures of about 220 degrees centigrade.
In still another embodiment of the present invention a supplemental heat source used to activate the solder paste may be supplied by one or more jets of hot gas which are directed toward the exposed areas of the substrate. Suitably, the jet of hot gas extends transversely over the width of the substrate.
In yet another embodiment of the present invention, a method for reflowing solder to interconnect a plurality of electronic components to a substrate is provided. The method includes inserting the substrate into an oven, preheating the substrate and the plurality of electronic components disposed thereon, providing additional heat energy to reflow the solder using a supplemental heat source positioned within the oven, creating a stream of hot gas using the supplemental heat source, wherein the gas flows transversely across the substrate, and supporting the substrate with a pallet, wherein the pallet absorbs heat from the substrate.